Usually you set max height and/or max width in pixels. When I'm downsizing photos I usually resize to max height 550 pixels (width automatic). That means I can see the entire photo on screen in my browser without having to scroll in any direction. To add photos to TE I make them smaller because the available space is less with the menus and whatnot, max height 300 pixels or so seems to work fine. If you right-click on any photo here you can see how many pixels they are each way.

If the file size is still too big (ie. above 195 Kb) you can either downsize them a bit more, or store in a different format, .gif files seem to take less space than .jpg.

Edit: I see you got it :-) Lovely dogs!